Usually, a reader writer system using the IC card is ordinarily referred to as a non-contact IC card system, and has been put to practical use in a physical distribution system, a transportation system, a goods management and book management system or the like using a frequency band of, for instance, 13.56 MHz. This system includes the IC card having an IC chip and an antenna coil on one card made of a resin and a reading and writing part for communicating with the IC card. The reading and writing part is provided with a loop antenna. The electric power and the transmit data are constantly or intermittently transmitted and the receive data is obtained from the IC card located within a range where the electric power and the transmit data can be received by this loop antenna.
As means for increasing a communication range of the usual reading and writing part, a consideration may be made that the size of one loop antenna is increased. However, when the size of the antenna is enlarged, the sensitivity of the antenna is increased. Thus, a problem arises that the influence of unnecessary noise from a peripheral part is liable to be received and the increase of the radiation of an electric field to a remote place greatly affects other electronic devices. Further, problems arise in view of an electric performance and a mechanical performance that the antenna itself is liable to be affected by structures such as neighboring metal owing to the increase of the antenna size, and the improvement of strength is required in terms of a mechanism, and as a result, a weight is increased.
Further, as another means for increasing the communication range, a contrivance is proposed that a closed loop circuit (a resonance circuit) is arranged in the vicinity of the loop antenna and electro-magnetically connected to the loop antenna so that a magnetic flux generated in the loop antenna passes the closed loop circuit (resonance circuit), and accordingly, the closed loop circuit functions like the loop antenna to widen the communication range.
As one example thereof, FIG. 19 shows a perspective view of an antenna unit in a related art described in (Patent Document 1). In FIG. 19, the antenna unit 101 has a loop antenna 111 provided on an antenna substrate 102 via a driver 107 connected to an oscillator 108 and closed loop circuits 112 and 121 arranged in the periphery thereof. The closed loop circuits 112 and 121 electro-magnetically connected to the loop antenna 111 by a magnetic flux 113 generated from the loop antenna 111 function like the loop antenna 111.
Patent Document 1: JP-A-2001-85927